What's a budget-friendly solution to resetting the ECU?

Hi. We just bought a secondhand 2008 Toyota Vios 1.3 E last March with a 35,000+km odometer reading. The car performed smoothly (it has complete casa maintenance records). I am from Zamboanga City, and the car is our means of daily commute.

The other day, the battery needed replacement, so we bought the same kind, a Motolite Gold NS60. Since the car was not starting anymore, the installation was performed at home. The problem came after installing the battery.

When I turned on the ignition to start the engine, the car started perfectly, but the engine began to shake because the rpm slowly went down to 600, 500 and then 300. The car then eventually stalled. I again started the engine. Starting was fine, but the rpm again went down and then the car stalled. It took only seconds to happen before the stalling. I called my mechanic friend and told him what happened; he told me to start the engine and detach the negative terminal for around 10-15 seconds, and then keep the foot on the throttle at high rev, around 3,000-4,000rpm--then attach the terminal again. It helped as the rpm steadied at 750-800rpm, but when I turned on the A/C, the rpm again fluctuated, falling below 500 before rising to 800-900 and then falling again to 500. So I just revved the engine until it became normal again.

One morning after bringing the children to school, I noticed that the rpm would again fluctuate from 500 to 800 (when slowing down or when stuck in traffic). My mechanic friend suggested that I bring the car to Toyota to reset the computer settings, so I inquired with the Toyota center (over the phone) regarding the problem, and I was told that the car needed throttle cleaning and new spark plugs, but it doesn't need resetting of the computer. It would have been okay for me, but the casa charges P5,000 for the labor.

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Is it too much? I feel it is somewhat overpriced. Is there a budget-friendly solution to this? Can I repeat the DIY resetting, pulling out the clamp on the negative terminal for a few seconds and then putting it back? Please help. Thank you and God bless.

Vincent Yra

Hi, Vincent. Your e-mail didn't indicate the scope of work that the dealership was charging you P5,000 for, but if it includes a tune-up and changing the oil, the oil filter and the air filter apart from the spark plugs and throttle body cleaning, it would be in the right ballpark. Given that the vehicle was purchased pre-owned, having it checked out by a qualified tech would be a good idea.

However, if you want to clean the throttle body and you're handy with tools, you may perform the following:

1. Remove your intake hose.

2. Moisten a clean rag with alcohol.

3. Wipe down the interior of your throttle body with the rag. Be careful not to get any debris or foreign objects into the exposed ports while you are doing so.

4. Once you are done, put everything back in order.

You can also try to remove the battery again (with the engine cold) and leave it disconnected for at least 15 minutes to an hour. Then reconnect the battery. Turn the ignition on, but don't start the car. Step on the accelerator pedal fully and then release it. Do it a few times before turning the ignition off. After turning the ignition off, restart the car.

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Alternatively, you can start the engine and let it run for a few seconds on its own before applying enough throttle to bring the engine speed to about 3,000rpm. Keep it there for a while until the radiator fan comes on a couple of times.

When you do this, keep all electrical accessories turned off. This includes the radio, lights and whatever else. After you're done with that, drive around with the A/C off for a while.

What you are trying to do is to have the ECU relearn the driving conditions it will be operating under and the proper idle rpm. Once the idle speed is relatively stable again, you can turn on the aircon again. There will most likely be fluctuation in the idle speed for a while, but bear with it until it becomes normal again.

For a lot of modern adaptive ECUs, removing the battery is akin to resetting the ECU. It erases the "fine tuning" of the default base program it has learned over the period of time since the last time the battery was disconnected. It's partly for this reason you don't want to run your battery down to the point that it won't start the car, as it will seem like that something got damaged. Unscrupulous mechanics sometimes use it as an excuse to "fix" the car.

Any idea what causes hard starting aside from the battery? What is the solution to this problem? Thanks in advance.

Jake S.

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Hi, Jake. I would check the electrical and fuel systems to make sure that they are correct for the engine. Being a swap, the electrical system--for the engine, ECU and sensors--should be engine-model-correct. Try to secure a workshop manual or electrical system diagram for the vehicle that the engine came from and follow it accordingly.

You can also put a fuel pressure gauge in the fuel system and check for proper fuel pressure with the engine running. If it is correct when the engine is running, keep the gauge on when you shut off the engine and observe if the fuel pressure drops quickly. The fuel line leading up to the fuel regulator may be emptying itself of fuel when the engine is off, and the hard starting might be due to not enough fuel getting to the engine in the first few moments.

I hope you also upgraded your suspension and brakes with the engine swap. Not doing so isn't a very good idea.